Field of the Invention
The present invention relates to an overcurrent protection function of a fixed on-time control type switching power supply device.
Description of Related Art
A fixed on-time control type switching power supply device has a feature that it can obtain higher load response characteristics with a simpler circuit structure than a fixed frequency control type switching power supply device (such as a voltage mode control type or a current mode control type). As one form of the fixed on-time control type switching power supply device, there is conventionally known a fixed on-time with bottom detection type switching power supply device (see, for example, JP-A-2010-35316).
FIGS. 8A and 8B are a circuit block diagram and an operation waveform chart illustrating a general example of a fixed on-time with bottom detection type switching power supply device. The fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A is a step-down type DC/DC converter, which steps down an input voltage Vin so as to generate a desired output voltage Vout. In the fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A, when it is detected that a feedback voltage Vfb drops to a reference voltage Vref, an upper-side transistor N1 is turned on so that a switch voltage Vsw becomes high level for a predetermined on-time Ton. Note that the upper-side transistor N1 is in OFF state during a period other than the predetermined on-time Ton. In addition, the upper-side transistor N1 and a lower-side transistor N2 are switched in a complementary manner.
When performing overcurrent protection in the fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A, a low-side sensing type overcurrent protection circuit is usually used. In the low-side sensing type overcurrent protection circuit, even if the feedback voltage Vfb drops to the reference voltage Vref during the on-time of the lower-side transistor N2, if an inductor current IL is larger than a bottom value threshold value THb, the upper-side transistor N1 is not turned on until the inductor current IL becomes the bottom value threshold value THb or less. Thus, in a state in which overcurrent may occur without overcurrent protection, a bottom value (minimum value) of the inductor current IL is adjusted to the bottom value threshold value THb.
Further, in the fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A, the upper-side transistor N1 is turned off after the predetermined on-time Ton elapses after the upper-side transistor N1 is turned on. Therefore, ignoring delay time generated in a comparator CMP1 or the like, in the state in which overcurrent may occur without overcurrent protection, the bottom value (minimum value) of the inductor current IL is the same as the bottom value threshold value THb, a peak value (maximum value) of the inductor current IL becomes the sum of the bottom value threshold value THb and a ripple component R of the inductor current IL (see FIG. 9). Note that the ripple component R of the inductor current IL is uniquely determined by a function of the predetermined on-time Ton, the input voltage Vin, and the output voltage Vout.
Therefore, in the fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A, an appropriate overcurrent protection can be achieved only by the low-side sensing type overcurrent protection circuit.
Here, the fixed on-time with bottom detection type switching power supply device illustrated in FIG. 8A may be additionally provided with a function of automatically extending the on-time of the upper-side transistor N1 for increasing on duty of the upper-side transistor N1 if the output voltage Vout at a time point when the predetermined on-time Ton elapses is lower than a specified value.
In the case where this function is additionally provided, when the on-time of the upper-side transistor N1 is extended to be longer than the predetermined on-time Ton, the ripple component R of the inductor current IL becomes larger than that when the on-time is not extended. Therefore, a high-side sensing type overcurrent protection circuit becomes necessary, which suppresses the peak value (maximum value) of the inductor current IL. The high-side sensing type overcurrent protection circuit turns off the upper-side transistor N1 at a time point when the inductor current IL becomes larger than a peak value threshold value THp during the on-time of the upper-side transistor N1.
However, there is a problem that if only the high-side sensing type overcurrent protection circuit is disposed so as to simply add the peak value threshold value THp, an operating frequency of the switching power supply device when the overcurrent protection operation is performed is changed from the operating frequency in the normal operation (in which the overcurrent protection operation is not performed). Specifically, as illustrated in FIG. 10, if a difference Dpb between the peak value threshold value THp and the bottom value threshold value THb is smaller than the ripple component R of the inductor current IL, the operating frequency of the switching power supply device in the overcurrent protection operation is increased from that in the normal operation. In addition, on the contrary, if the difference Dpb between the peak value threshold value THp and the bottom value threshold value THb is larger than the ripple component R of the inductor current IL as illustrated in FIG. 11, the operating frequency of the switching power supply device in the overcurrent protection operation is decreased from that in the normal operation. Note that broken lines illustrated in FIGS. 10 and 11 indicate the same waveform as that of the inductor current IL illustrated in FIG. 9, having the same frequency as the operating frequency of the switching power supply device in the normal operation.